The present invention relates to a method of forming the edge of an ophthalmic lens, and in particular to a method of bevelling ophthalmic lenses. The invention also relates to a machine for grinding ophthalmic lenses.
As is known, bevelling is an operation which involves forming a rib or bevel, generally of triangular cross-section, on the peripheral edge of an ophthalmic lens. This rib or bevel is to be engaged in a groove, generally called a bezel, in the ring or surround of a spectacle frame in order to retain the lens therein.
Usually, the bevelling operation is preceded by a trimming or routing operation in which the superfluous peripheral part of the ophthalmic lens is removed to match the contour of the lens periphery, which is, in general, circular, to that of the circle or surround of the spectacle frame in which the lens is to be mounted.
Usually, these trimming and bevelling operations are carried out in succession on one grinding machine which is equipped with a set of suitable grinding wheels.
In practice, such a grinding machine comprises a machining station equipped with at least one trimming wheel and with at least one bevelling wheel, each of said wheels being rotatable by way of a drive motor, and a carriage which carries spindles extending parallel to the axis of said grinding wheels. The spindles are arranged to axially clamp the lens and they are mounted for rotation by way of a further drive motor. The carriage is mounted on a chassis for movement transversely relative to the axis of the grinding wheels and for movement axially parallel to the axis of these grinding wheels. Suitable control means for controlling the movement of the carriage are provided.
The transverse displacement of the carriage relative to the axis of the grinding wheels is necessary to bring the ophthalmic lens into contact with the wheels. This displacement can be enabled, for example, by pivotably mounting the carriage on a shaft extending parallel to the axis of the grinding wheels or by mounting the carriage such that it is movable perpendicularly to the axis.
Preferably, the carriage is biassed towards the axis of the grinding wheels either by gravity, or, if appropriate, by elastic means.
The axial displacement of this carriage enables the ophthalmic lens to be brought successively to each of the different grinding wheels.
The trimming operation, which is intended simply to form a cylindrical edge on the ophthalmic lens, is usually carried out under the control of a template. At present the contour of the template corresponds to the contour of the ring or surround of the spectacle frame in which the lens is to be mounted. The template is fixedly mounted on one of the spindles clamping the ophthalmic lens, and is arranged to cooperate with a key which limits the transverse penetrating movement of the trimming wheel relative to the lens.
During the trimming operation, the edge of the ophthalmic lens is kept in contact with the trimming wheel. Whatever the characteristics of the ophthalmic lens, it is generally sufficient to rotate the lens about its own axis which is arranged to extend parallel to the axis of the trimming wheel.
The bevelling operation is not so easily carried out, especially as there is a need to take the particular curvature of one and/or the other of the faces of the ophthalmic lens into account. This is particularly true for so-called progressive lenses where the front face of the lens is not spherical and the lens has a vertical meridian along which the power is progressively variable.
It is, of course, important that the bevel formed during the bevelling operation should be on the actual edge of the lens between the angles of its periphery.
It is therefore necessary to displace the lens parallel to its axis during its rotation, so that its point of contact with the bevelling wheel follows a suitable path between the said angles.
Thus, in order to enable the particular curvature of the ophthalmic lens to be taken into account, the lens must be capable of being displaced parallel to its axis during its rotation.
Such axial or lateral displacement of the lens can be carried out manually. However, it is then necessary for the operator to have a certain dexterity as the positioning of the lens is carried out visually.
Consequently, the result is always relatively approximate.
Alternatively, the ophthalmic lens can be laterally displaced by way of a guide wheel with a double slope defining a groove into which the entire edge of the lens penetrates. The guide wheel therefore automatically centres the lens permanently.
However, where ophthalmic lenses having thick edges are to be treated, the width which such a double-slope bevelling wheel must possess can quickly become prohibitive.
Moreover, to ensure that the lens can be freely displaced axially, it is necessary for the spindles clamping the lens to be perfectly balanced. This implies that the stand or chassis on which the spindles are mounted must be placed at a suitable level.
Consequently, it has been proposed in automatic grinding machines that the axial displacement of the ophthalmic lens to be bevelled should be controlled by control means such that the point of contact of the lens with the bevelling wheel follows a specific path.
French published Patent Application No. 2,481,635, describes an apparatus in which the key associated with the template is mounted so as to be transversely movable relative to the support spindles under the control of a control unit. This control unit provides a limited number of possible bevel paths, for example three or four, from which the operator can select the one which appears to him to be the most suitable.
However, as previously, it is necessary for the operator to be a specialist.
French published Patent Application No. 2,475,446, describes an apparatus by which bevelling is carried out in two stages. In the first stage, which is a pre-bevelling operation carried out by means of a double-slope bevelling wheel, the control unit detects the axial displacement of the lens during the bevelling operation as a function of its angle of rotation about its axis and records the path followed by the bevel of this lens. The axial displacement of the lens takes place freely for the reasons mentioned above.
In the second stage, which is a bevelling operation for finishing the lens, the control unit systematically ensures that the axial displacement of the lens corresponds to the previously recorded bevel path.
Of course, if the pre-bevelling operation, from which the bevel path has been recorded, has not been correctly performed, the bevel finally produced is not satisfactory.
In addition, and as previously, it is necessary to use a double-slope bevelling wheel which of course has the abovementioned disadvantages as regards bulk.